The inner medullary urinary concentrating mechanism is an important yet poorly understood renal function. The goal of this project is to advance our understanding of the relationships between structural organization and function, including fluxes of inorganic ions, urea, and water, and the urine concentrating mechanism in the mammalian renal medulla. Three-dimensional architecture underscores the paradigm that the inner medullary interstitium is not one single well-mixed compartment. Incomplete knowledge of loop of Henle transepithelial NaCl, urea, and water permeabilities is the single most critical barrier to creating advanced models that explain generation of the IM osmotic gradient and the UCM. The UCM can only be fully understood by taking into consideration three-dimensional structural and functional architecture of the renal IM. Based on the work of others and on our previous work in determining the functional organization and solute and water fluxes, the three following specific aims will be investigated. Aim 1, determination of water, urea, and NaCl permeabilities of IM loops of Henle from moderately-concentrating rats. Aim 2, determination of functional architecture of loops of Henle and CDs, vascular networks, and interstitial nodal spaces (INSs) in the IM. Aim 3, determination of urea, and NaCl permeabilities of loops of Henle from diuretic and antidiuretic rats, and following vasopressin treatment in vivo and in vitro. Experimental approaches will include: 1) production of three-dimensional reconstruction of all IM thin limbs of Henle's loops, CDs, and vasa recta from serial sections, using immunocytochemical markers of physiological function to identify tubule and vessel segments, sites of cellular transport functions, and interstitial compartments and determination of structure-to-structure interactions, and 2) direct measurements, by in vitro microperfusions, of the NaCl, urea, and water permeabilities of specific thin limb segments defined in the reconstructions. Tubule permeabilities will be investigated with and without vasopressin in vitro, and permeabilities will be investigated following water diuresis, antidiuresis, and exposure to vasopressin in vivo.